The present invention provides glasses which combine the optical characteristics of high refractive and dispersive indices with physical properties of low density and good chemical durability. This combination of properties is particularly required in such applications as high correction ophthalmic lenses and space optics where the weight factor becomes especially significant.
Glasses having a high refractive index and high dispersive index, also known as Abbe number, have been known at least since 1939 when U.S. Pat. No. 2,150,694 was granted to G. W. Morey. However, such glasses have normally employed relatively large contents of heavy metal oxides which contribute to a glass of high density. Recently, product applications have developed wherein light weight is of paramount importance. Thus, in space optics for example, it is necessary to optimize the ratio of load/power in rockets used in space experiments. Also, in ophthalmic lenses it is of interest to reduce lens weight and thereby provide more comfort to the wearer.